1. Field of the Invention
This invention pertains to connection admission control in a telecommunications system, and particularly to connection admission control for such systems when AAL2 protocol signaling is employed.
2. Related Art and Other Considerations
Asynchronous Transfer Mode (ATM) is becoming increasingly used in communication networks. ATM is a packet-oriented transfer mode which uses asynchronous time division multiplexing techniques. Packets are called cells and have a fixed size. An ATM cell consists of 53 octets, five of which form a header and forty eight of which constitute a “payload” or information portion of the cell. The header of the ATM cell includes two quantities which are used to identify a connection in an ATM network over which the cell is to travel, particularly the VPI (Virtual Path Identifier) and VCI (Virtual Channel Identifier). In general, the virtual path is a principal path defined between two switching nodes of the network; the virtual channel is one specific connection on the respective principal path.
Between termination points of an ATM network a plurality of nodes are typically situated, such as switching nodes having ports which are connected together by physical transmission paths or links. The switching nodes each typically have several functional parts, a primary of which is a switch core. The switch core essentially functions like a cross-connect between ports of the switch. Paths internal to the switch core are selectively controlled so that particular ports of the switch are connected together to allow a cell ultimately to travel from an ingress side of the switch to an egress side of the switch.
A protocol reference model has been developed for illustrating layering of ATM. The protocol reference model layers include (from lower to higher layers) a physical layer (including both a physical medium sublayer and a transmission convergence sublayer), an ATM layer, and an ATM adaptation layer (AAL), and higher layers. The basic purpose of the AAL layer is to isolate the higher layers from specific characteristics of the ATM layer by mapping the higher-layer protocol data units (PDU) into the information field of the ATM cell and vise versa. There are several differing AAL types or categories, including AAL0, AAL1, AAL2, AAL3/4, and AAL5.
AAL2 is a standard defined by ITU recommendation I.363.2. An AAL2 packet comprises a three octet packet header, as well as a packet payload. When desired, plural AAL2 packets can be inserted into a standard ATM cell.
ATM AAL2 is the standardized transmission technology for a radio access network known as the UMTS Terrestrial Radio Access Network (UTRAN). In the UTRAN, a switched AAL2 network interconnects radio base station (RBS) nodes (also more recently known as Node B or B-node) and radio network controller (RNC) nodes, and further connects the whole radio access network to the core network via the Iu interface.
Typically, telecommunications networks are conceptualized as having certain layers, such as a call layer and a transport layer. In a telecommunications network such as the UTRAN, nodes of the network through which a connection is to be routed have a connection admission control function which determines, during attempted set up or modification of the connection, whether transport network resources in a transport layer can be successfully allocated for the sought connection. It is quite important to have a connection admission control function that allocates transport network resources in a reliable way. “Reliable” means that transport resources are allocated such that there is a high probability that the quality of service (QoS) requirements for the connection can be met.
In the UTRAN, a signaling protocol known as AAL2 Signaling is used to control the establishment of the AAL2 connections. The AAL2 Signaling includes two basic capability sets, known as AAL2CS1 and AAL2CS2 [respectively described in ITU-T Recommendation Q.2630.1 AAL Type 2 Signalling Protocol (Capability Set 1) and ITU-T Recommendation Q.2630.1 AAL Type 2 Signalling Protocol (Capability Set 2)]. One such signaling message of the AAL2 protocol (Establish Request) travels through the AAL2 switching network and carries (e.g., includes) certain information such as the destination address of the message and the resource requirement of the new connection. Each intermediate AAL2 node consults the Establish Request signaling message to check the resource requirement of the new connection. Such nodes then execute a connection admission control algorithm or function to determine whether those nodes can accommodate the new AAL2 connection. If the decision is positive the signaling message proceeds towards the remote destination endpoint.
In the AAL2 protocol Establish Request message, a parameter known as the link characteristics parameter carries the resources requirement-related information. This information is basically a traffic descriptor. This traffic descriptor is basically intended to characterize the behavior of the source that will load the new AAL2 connection. A key to reliable utilization of the network resources is the precision of this traffic descriptor. Currently, the link characteristics parameter has four fields which describe the AAL2 source that will load the new AAL2 connection. These four fields are: (1) Maximum CPS-SDU bit rate; (2) Average CPS-SDU bit rate; (3) Maximum CPS-SDU size; and (4) Average CPS-SDU size.
In AAL2 Signaling CS2 [AAL2CS2], there is an option to specify also the quality of service (QoS) requirement of the AAL2 flow. This option specifies the type of underlying ATM VCC, which should be used to carry that particular AAL2 connection. Two QoS classes are supported, stringent and tolerant. The definiition of these ATM QoS cases can be found in ITU-T Recommendation I.356 B-ISDN ATM Layer Cell.
A connection admission control algorithm using the above listed four fields to characterize the traffic can result in very conservative or very optimistic allocation of network resources, depending on the values of the parameters. Connection admission control based on these four fields alone can lead to very low network utilization or overloads.
What is needed, and an object of the present invention, is a technique for more precise characterization of AAL2 traffic in order to enable more reliable connection admission control methods to run in AAL2 nodes.